Diolefin production



p 1947- J. A. SAMANIEGO El AL 2,427,925

DIOLEFIN PRODUCTION Filed Oct. 10, 1944 Lwtow ummnzt lnvenfogs: dose A- Samaniago M011 Souders Jn Patented Sept. 23, 1947 DIOLEFIN PRODUCTION l I Jos A. Samaniego, San Francisco, and Mott Souders, Jr.,

Piedmont, Calif., assignors to Shell Development Company, San Francisco, Calif., a corporation of Delaware Application October 10, 1944, Serial No. 558,092

4 Claims. 1

This invention relates to an improved process for the separation of diolefins from hydrocarbon mixtures containing them and relates more particularly to an improved process for the more efficient production of substantially pure diolefins from hydrocarbon mixtures containing these diolefins in admixture with saturated and/or unsaturated hydrocarbons of similar boiling range. A specific embodiment of the invention relates to an improved process for the separation of substantially pure isoprene and piperylene from hydrocarbon fraction containing them.

The diolefins are finding ever increasing application as starting and intermediate materials in the production of a great number of products such, as elastomers, resins, textiles, lacquers, dye-stuffs and related industries. Utilization of the diolefi-ns in many of these industries has been handicapped by the inability to effect the complete and efficient separation, on a practical scale, of these hydrocarbons from other hydrocarbons of similar boiling range in admixture with which they are generally encountered. A principal source of the diolefins resides in the hydrocarbon mixtures procluced by the pyrogenic or catalytic treatment of hydrocarbons or hydrocarbon mixtures of greater saturation than the diolefins. The resulting cliolefin-containing hydrocarbon products, as i well known, consist of highly complex mixtures from which the separation of individual components such as, for example, a desired diolefin, by such methods as distillation, solvent extraction, etc., is often highly impractical if not impossible. Resort to separation by heretofore available means involving chemical reactions has often been exceedingly impractical due tothe complexity of the operative steps involved and often because of the difiiculties encountered in effecting the separation of diolefins from other unsaturated hydrocarbon by such means.

An advantageous method heretofore disclosed for the separation of diolefins from admixed hyrocarbons of similar boiling range makes use of the [formation of the sulfone of the diolefin. Thus, it has been previously known that diolefins such as, for example, butadiene, isoprene, piperylene, etc., react with sulfur dioxide to form sulfones and that the diolefins are liberated when thesulfone is heated. Production of the diolefins in the high degree of purity essential to their eflicient utilization in a great many processes is, however, seriously handicapped and often rendered impossible by the difficulties involved in effecting a practical recovery of a diolefin free of any substantial amount of other hydrocarbons by such methods as disclosed heretofore. These difficultiesinherent in these processes are attributable to the fact that the diolefin sulfone decomposes at a relatively low temperature, thereby rendering exceedingly difficult the separation, on a practical scale with a minimum of operative steps, of the hydrocarbons other than the desired diolefin generally unavoidably admixedv therewith. Further difiicultie inherent in these processes lie in the loss of diolefin due to the fOrmation of polysulfones and in the ease with which the diolefin and the sulfur dioxide recombine under conditions which must be resorted to to effect their separation toany substantial degree by such means as fractionation.

It'is an object ofthe present invention to provide an improved process for the more efficient separation of diolefins from hydrocarbon mixtures containing them.

Another object of the invention is the provision of an improved process for the more efficient separation of diolefins from hydrocarbon fractions of relatively narrow boiling range.

A more particular object of the invention is the provision. of an improved process for the more efficient separation of piperylene and isoprene from hydrocarbon mixtures comprising these diolefins in admixture with other hydrocarbons of similar boiling range.

Another object of the invention is the provision of an improved process for the more efiicient production of isoprene and/or piperylene, free of any substantial amount of other hydrocarbons, [from hydrocarbon fractions predominating in hydrocarbons having five-carbon atoms to the molecule.

In accordance with the present invention, a hydrocarbon mixture containing at least one diolefin such as, for example, a diolefin-containing hydrocarbon fraction obtained by the pyrogenic or catalytic treatment of aliphatic hydrocarbons, is contacted with sulfur dioxide in a reaction zone under conditions effecting the interaction of the sulfur dioxide and diolefins with the formation of the corresponding diolefin sulfones. Unreacted hydrocarbons and sulfur dioxide are separated from the sulfones in a two-stage operation comprising an initial flash vaporization of a substantial amount of unreacted hydrocarbons and sulfur dioxide from the sulfone in a vaporizing zone and the subsequent removal or substantially all remaining unreacted hydrocarbons from the sulfone by means of stripping with sulfur dioxide, obtained within the system, in a sulfone stripping zone maintained under conditions avoiding the decomposition of th sulfones a well as the formation of polysulfones. The sulfone, now free of any substantial amount of unreacted hydrocarbons, is decomposed in a sulfone decomposing zone under conditions resulting in the formation of substantially only diolefin and sulfur dioxide. Effluence from the sulfone decomposing zone is contacted with a selective solvent for the diolefin in a diolefin extraction zone to effect the separation of the diolefin from the sulfur dioxide under conditions eliminating the difficulties arising from the facility with which sulfur dioxide and the diolefins recombine. The resulting diolefins, free of any substantial amount of sulfur dioxide and hydrocarbons other than the desired diolefins, are separated from the system as the final products. The relatively pure sulfur dioxide, eliminated from the extraction zone, is passed to the sulfone stripping zone to be used therein as the stripping medium. Vapors, comprising unreacted hydrocarbons and sulfur dioxide, from the vaporizing and sulfone stripping zone are treated to effect the removal of at least a substantial amount of the unreacted hydrocarbons therefrom and are then recycled to the reaction zone.

In order to set forth more fully the nature of the invention it will be described in detail herein in its application to the production of isoprene and piperylene from hydrocarbon mixtures containing these diolefins, with reference to the attached drawing forming a part of the present specification and wherein the single figure illustrates one form of apparatus suitable for executing the process of the invention.

A suitable charge material for the process of the invention is a diolefin-containing hydrocar- Icon fraction such as, for example, a hydrocarbon fraction consisting predominantly of unsaturated hydrocarbons having five carbon atoms to the molecule, as obtained in the fractionation of the distillate products of the high temperature cracking in the vapor phase of naturally occurring hydrocarbon mixtures or fractions thereof. The charge is preferably fractionated to obtain an acyclic diolefin concentrate containing substantially all of the isoprene and piperylene. The acyclic diolefin concentrate is preferably freed of any substantial moisture content, for example, by passage through any suitable drying agent. The resulting dry acyclic diolefin-containing hydrocarbon mixture is passed through line Ill into a suitable reaction zone. The reaction zone may comprise a reactor of enlarged cross sectional area I I. Although but one such reactor is shown in the drawing, a plurality of such reactors connected in series or in parallel may suitably be used. Sulfur dioxide, recycled from within the system through line I2 as described more fully herein below, is introduced into line II] to mix are, however, preferably carefully controlled to.

effect the reaction of sulfur dioxide with isoprene and piperylene with the formation of isoprene sulfone and piperylene sulfone as the predominant reaction". Particularly advantageous results are obtained with the use of a temperature of about (3., though somewhat higher or lower temperatures may be employed. It is to be stressed that the sulfones are ordinarily decomposed at relatively low temperatures and that therefore temperatures within the reactor are preferably controlled to remain below the temperature at which any substantial decomposition of sulfones is encountered. Maintenance of a substantial excess of sulfur dioxide in the reaction zone enables the utilization of higher reactipn temperatures than would be possible in the dbsence of such excess of sulfur dioxide, Pressures within reactor H are preferably sufficiently high to maintain at least a substantial part of the reactants in the liquid phase. As a general rule the pressure within reactor I I is preferably maintained in excess of about pounds per square inch and still more preferably in excess of about 350 pounds per squareinch. Temperature conditions within reactor Ii are controlled by the passage of vaporized reactants from reactor II into suitable cooling means, for example, a condenser I4, wherein latent heat of vaporization is withdrawn from the vapors, and return of the resulting condensate to the reactor. Under the above-defined reaction conditions isoprene and piperylene in the hydrocarbon charge react with sulfur dioxide to form isoprene sulfone and piperylene sulfone respectively in the absence of any substantial side reactions such as, for example, olefin sulfonation.

As pointed out above, it is essential to the attainment of a product of the desired purity that substantially all of the unreacted hydrocarbons other than isoprene and piperylene in admixture with the sulfones be separated therefrom. subjection of the reaction products to such means as simple fractionation to effect such complete separation requires the need of substantial additional heat input into the reaction products at a temperature level exceeding by far the maximum reactor temperature generally employed. Since the sulfones are decomposed at relatively low temperatures, such treatment, resulting in a reversal of the sulfone producing reaction in the fractionating zone, would render the system highly impractical. Thus, isoprene sulfone, for example, undergoes decomposition at a temperature substantially below its boiling temperature. These difficulties are overcome in the process of the invention by the utilization of a two-step separation wherein unreacted hydrocarbons are separated from the sulfones at temperatures which are generally lower than the reaction temperature, thereby obviating any tendency towards sulfone decomposition. The method of separation inherent in the process of the invention not only avoids the decomposition of sulfones but enables substantially complete separation of unreacted hydrocarbons from the diolefin sulfonesproduced.

To effect the separation of the unreacted hydrocarbons from the sulfones, efiluence from reactor I! is passed through valved line [6 into a vaporizing zone comprising, for example, enlarged chamber I]. By a suitable reduction of pressure and controlled heat input by means of closed coil I8, a substantial part of the unreacted hydrocarbons and sulfur dioxide are separated from the diolefin sulfones in vaporizer H. The diolefin sulfones and remaining sulfur dioxide and unreacted hydrocarbons, are forced through line 19 into a stripping zone comprising, for example, a stripping column 20. Within stripping column. 20; the suliones are stripped with sulfur dioxide introduced into the lower part of the. column by means of line 22, thereby eifecting the removal of substantially all of the remaining unreaoted hydrocarbons from the diolefin sulfones.

Temperatures within vaporizing chamber l1 and stripping column 20 are maintained, substantially at and preferably below the temperature maintained in the reactor H. Thus, the temperature in vaporizer I"! and stripping column 2!! is prererably' maintained.- below about 100 C. Maintenance of a temperature of about 90 C. and a pressure of about 100 pounds per square inch in vaporizer I! and stripping column 2.0 is found to be: highly advantageous. Any heat input into stripping column 20, required toaid in effecting the desired separation, is accomplished by the passage of bottoms from the column through a heater 23 and thence back into the column. Since it is essential that any substantial increase in temperature of the diolefin sulfones be avoided; heater 23 is of a type enabling the rapid passage of liquid therethroughto result in but a moderate increase in temperature. It is found that a temperature gradient substantially in excess of about 5 C; from inlet to outlet of heater 2-3- is generally not needed and is therefore to be avoided. Under the above defined two-stage separation the hydrocarbon content of the sulfones taken from the lower part of stripper 20 is readily maintained at an unusually low level, for example, at about 0.1 mol per cent and less.

Vapors eliminated from vaporizer l1 comprising sulfur dioxide and unreacted hydrocarbons are. passed through line 26 to a fractionator 21. Suliiur dioxide containing unreacted hydrocarbons, eliminated overhead from stripping column 20 is passed through line 28 into line 25; Within fractionator 21 there is separated a vapor fraction COIIIDIiSillg sulfur dioxide and a minor amount of unreacted hydrocarbons from a liquid fraction comprising unreacted, hydrocarbons. The. liquid fraction is eliminated from the system through valved line 28. Due to the tendency of isopentane and pentane to form azeotropes with sulfur dioxide. the vapor overhead of column 2! will com:- prlse a mixture. generally consisting of sulfur dioxide containing from about 12% to about of pentanes.

The vapor overhead from column 21 is passed through line 3.0 and condenser 31 into a, receiver 3-3.. Liquid comprising, sulfur dioxide and a minor amount of saturated. hydrocarbons is, taken from receiver 33 and forced through line, I2 into line H! leading to; the reaction. zone. If desired, the hydrocarbon content of the sulfur dioxide stream passing through line l2. may be reduced. BY suitable adjustn-rentv of valves 34, 35. and 36 at least a part, of the. stream may be diverted from line. I2. through cooler 31, into av separator 38. In passing through cooler 31 the stream is cooled to a temperature sufiiciently low to. effect the separation within cooler 38 of a supernatant hydrocarbon layer from a lower layer consisting of sulfur dioxide having a lower hydrocarbon content than the liquid in accumulator 33. Cooling the stream flowing through cooler 31 to, a temperature of about 10 C. results in reduction. of thehydrocarbon content of the lower layer in separator 4.0 to. about 7% by weight. The lower layer is passed through valved line 39 into. line I 2. The supernatant hydrocarbon layer containing a minor amount of sulfur dioxide istaken from. separator 38 and passed through line: 450 to fractionator 21'. In this wise the hy- 6 drocarbons eliminated from the recycling sulfur dioxide stream are ultimately eliminated from the system, through valved line 28. vA valved line 4'! is provided, for the introduction of make-up sulfur dioxide into the system.

Liquid consisting of isoprene sulfone and piperylene sulfone is continuously passed from heater 23 through line 45 into a sulfone decomposing zone. The sulfone decomposing zone may suitably comprise an enlarged chamber 46 provided with suitable heating means such as, for example, closed heating coil 41. Within decomposer 46 the sulfones are raised to a temperature sufficiently high to effect their decomposition resulting in the liberation of the diolefins. Maintenance of temperatures in excess of about C. and preferably in the range of, for example, from about C. to about 175 C. and a pressure,. for example, in the range of from about 15 pounds to about 300 pounds and preferably in the range or from about 100' pounds to about pounds per square inch within the decomposer 4-6 have been found advantageous. Under the above defined conditions isoprene sulfone is decomposed to isoprene and sulfurdioxide, and piperylene sulfone is decomposed to piperylene and, sulfur dioxide. In order to avoid the formation of undesired reaction products, for example, materials comprising polysulfones, unnecessarily high temperatures and a residence time longer than that essential to the obtaining of the desired dienes are avoided in decomposer 46. From chamber lfi decomposition products consisting of the dienes and sulfur dioxide are passed through line 5% into an extraction column 5!. Within extraction column 5| the products are contacted countereurrently with a selective solvent for the dienes such as, for example, a parafiinic hydrocarbon, preferably one having at least seven carbon: atoms tothey molecule. A suitable solvent comprises octanes. Although paraflinic hydro.- carbons' have been chosen as preferred solvents for selective removal of the dienes from the sulfur dioxide admixed therewith, the invention is in no wise limited to the utilization of any specifio solvent and any solvent capable of effecting the selective removal of the dienes may be used. Thus, such suitable selective solvents for the dienes comprise, for example, the hydrocarbons having a greater degree of saturation than the dienes extracted therewith or a mixture predominating in one or more such hydrocarbons, etc.

Rich solvent containing isoprene and piperylone is passed from the lower part of extraction column 54 into a soaking zone. The soaking zone consists of an enlarged chamber 53 provided with suitable: heating means such as, for example, a coil 54. Within soaker 53 the rich solvent containing the dio-lefins is maintained for a period of time at a sufficiently high temperature to assurethe decomposition of any diolefin sulfones which may haveformed during the extractive operation and also to prevent the further combination of any small amounts of sulfur dioxidestill present with the diolefin product. Vapors. from soaker 53 consisting of diolefins, octanes and any sulfur dioxide which may have entered into soaker 53 are returned to column 5! through valved line 55. Temperatures within the soaker may advantageously be maintained in the range of. for example, from about 120 C. to about C. At times it may be desirable to maintain a temperature within soaker 5% excess; of that maintained in the bottom of column To this intent, line 52 is provided with a pump 57 and line 55 with a valve 58 to enable the maintenance of higher pressures within soaker 53 than those maintained within column 5|.

Liquid comprising solvent, isoprene and piperylene is passed from soaker 53 through valved line 69 into a still 61. Within still El there is separated a vapor fraction comprising the isoprene and piperylene from a liquid fraction comprising the solvent. The solvent is returned to column 5! by means of line 62. A cooler 63 is provided in line 62 to eiiect the desired cooling of the solvent prior to introduction into column 51. A valved line 64 is provided for the introduction of make-up solvent into line 62. The vapor fraction is passed overhead from column 6! through line 65 and condenser 86 into an accumulator 61. From accumulator 6'! liquid comprising isoprene and piperylene is passed to a neutralizing zone by means of line 68. The neutralizing zone may comprise a contacting chamber 69 wherein the hydrocarbon stream is contacted with a suitable neutralizing agent such as, for example, an aqueous sodium hydroxide solution or the like, and a settling chamber '13 wherein separation of treating agent from hydrocarbons is effected. It is to be understood that the invention is in no wise limited by the type of neutralizing system employed and any conventional method for removing traces of remaining sulfur dioxide from the diolefin product may be resorted to. From separator it the diolefins are passed through line H into a fractionating column l2. Within fractionator 12 there is separated a vapor fraction consisting essentially of isoprene from a liquid fraction consisting essentially of piperylene. Liquid comprising piperylene is withdrawn from the lower part of column l2 by means of valved line 13. Vapors comprising isoprene are takenoverhead from column 12 and passed through line 74, condenser 75, into accumulator 16. Liquid comprising isoprene is withdrawn as a final product from accumulator '16 through valved line 11. v

The sulfur dioxide separated from the sulfone decomposition products in extraction column 51 is passed overhead therefrom through line 22 into the lower'part of column 20 to function as the stripping agent therein. A valved line 18 is provided for the introduction of additional sulfur dioxide into line 22, should this be required. It is to be pointed out that a highly important feature of the invention, contributing material- 1y to the increased eflicicncy with which diolefins can be separated. from hydrocarbons admixed therewith, resides in the utilization of sulfur dioxide emanating only from the sulfone decomposing zone as the stripping agent in the sulfone stripping zone, and the recycling to the reaction zone wherein the sulfones are formed, of sulfur dioxide emanating only from the vaporizing and sulfone stripping zones.

Since impurities in the charge, particularly oxygen and organic peroxides are detrimental to eflicient execution of the processfthe hydrocarbon feed is preferably subjected to any suitable pretreatment which may comprise such steps as. for'example, distillation over caustic followed by passage through suitable drying agents.

The detailed illustrative description of the invention has been directed to the separation of both isoprene and piperylene from a hydrocarbon mixture containing them. It is to be stressed,

however, that by suitable control of conditionsin reactor l l but one of the diolefins may be separated from the feed to the process. Thus, by limiting the time of contact in reactor I I the sulfone reaction product can be made to consist predominantly of isoprene sulfone even though piperylene is present in the charge. The process of the invention is furthermore not limited to the separation of only diolefins having five carbon atoms to the molecule but is applicable broadly to the separation of other diolefins such as, for example, 2methyl-1,3-pentadiene, from hydrocarbon mixtures containing them.

The following example consisting of one specific continuous operation is given as illustrative of the method of separating diolefins from admixture with hydrocarbons of similar boiling range in accordance with the process of the invention.

Example An unsaturated hydrocarbon fraction, obtained by fractionation of the distillate products of the vapor phase cracking of gas oil, consisting predominantly of unsaturated hydrocarbons having five carbon atoms to the molecule is distilled over caustic, dried, and again fractionated to separate therefrom a C5 diolefin concentrate having the following composition:

Mol per cent Cs paraflins 3.3 C5 olefins 31.9

Isoprene 40.3 Piperyline 24.5

The diolefin concentrate is mixed with S02 in a molar ratio of S02 to total dienes of 5:1 and is passed into the sulfone reactor maintained at a temperature of 99 C. and a pressure of 400 pounds per square inch. Reactor efiluence having a. composition of Mol per cent C5 parafiins 12.9 C5 olefins 7.6 Isoprene 0.5 Piperylene 1.5 S02 63.9

Sulfones 13.6

is passed into the vaporizer maintained at C. and 100 pounds per square inch wherein S02 and hydrocarbons are flashed off. The sulfone Mol per cent S02 68.1 Isoprene 21.3 Piperylene 10.5

High boiling material 0.1

is passed into the diene extraction column wherein it is contacted with a countercurrent flow of hot acid octanes. Rich octane solvent containing absorbed isoprene and piperylene is passed from the absorber to the seeker maintained at a temperature of 142 C. S02 eliminated overhead from the extraction column is passed to the sul- 9. fone stripper. S02 eliminated from the vaporizer and the sulfone stripper are combined and the resulting mixture, containing 22.6 mol per cent of C5 hydrocarbons, is passed into the S02 fractionator wherein an overhead product consisting of S02 containing 13.5 mol per cent C5 hydrocarbons is separated and recycled to the reactor. Liquid is drawn from the soaker and distilled to separate isoprene and piperylene. The resulting dienes are caustic treated and fractionated to separate piperylene of a purity of 90% from isoprene of a purity of 99%. The isoprene and piperylene thus obtained constitute 85% and 72% respectively of the isoprene and piperylene charged to the system.

We claim as our invention:

1. In a process for recovering isoprene and piperylene from a hydrocarbon mixture containing isoprene and piperylene in admixture with other hydrocarbons of a greater degree of saturation than said isoprene and piperylene wherein said hydrocarbon mixture is contacted with sulfur dioxide in a reaction zone under conditions resulting in the formation of reaction products predominating in diolefin sulfones consisting essentially of isoprene sulfone and'piperylene sulfone in admixture with sulfur dioxide and unreacted hydrocarbons and said reaction products are heated in a decomposition zone at a temperature sufficiently high to decompose diolefin sulfones with the liberation of isoprene and piperylene, the steps which comprise passing reaction products from the reaction zone into a vaporizing zone, separating a vapor fraction comprising sulfur dioxide and unreacted hydrocarbons from a liquid fraction comprising diolefin sulfones, sulfur dioxide and residual unreacted hydrocarbons in said vaporizing zone, stripping said liquid fraction with sulfur dioxide in a stripping zone, thereby removing substantially all residual unreacted hydrocarbons from said diolefin sulfones in the absence of side reactions to any substantial degree, and passing diolefin sulfones free of any substantial amount of unreacted hydrocarbons from the stripping zone to the decomposition zone, contacting the eilluence from said decomposition zone comprising isoprene, piperylene and sulfur dioxide with a countercurrent stream of a selective solvent for said diolefins in an extraction zone, thereby selectively absorbing isoprene and piperylene in said solvent, separating isoprene and piperylene from the resulting rich solvent containing isoprene and piperylene, and passing sulfur dioxide from said extraction zone to said stripping zone.

2. In a process for recovering an acycylic diolefin from a hydrocarbon mixture containing said acyclic diolefin in admixture with other hydrocarbons of a greater degree of saturation than said diolefin wherein said hydrocarbon mixture is contacted with sulfur dioxide in a reaction zone under conditions resulting in the formation of reaction products predominating in the sulfone of said acyclic diolefin in admixture with sulfur dioxide and unreacted hydrocarbons and said reaction products are heated in a-decomposition zone at a temperature sufiiciently high to decompose said diolefin sulfone with the liberation of said diolefin, the steps which comprise passing reaction products from the reaction zone into a vaporizing zone, separating a vapor fraction comprising sulfur dioxide and unreacted hydrocarbons from liquid fraction comprising diolefin sulfone, sulfur dioxide and residual unreacted hydrocarbons in said vaporizing zone, stripping said liquid fract on w th-sulfur. diox dein a s ri pin one. thereby remitting substantia l al residua unreacted nydres rben frqm -s 1 d le n u fonein absence of side r as i n o an substantial de ree, an pas ing diolefin sul ne f e o s b tanti l am nt of react d h ro ca bons f em the Si pins zon to the ades position z nacentacfins the efiiiisns f om he decomposition zone comprising diolefin s l.- fur vdioxide a cguntercurrent stream of selective solventfor'said'diolefin, thereby selectively absorbing said diolefin in the solvent, separating said diolefin from the resulting rich solvent containing said diolefin, and passing sulfur dioxide from the extraction zone to the stripping zone.

3. In a process for recovering isoprene from a hydrocarbon mixture comprising isoprene in admixture With hydrocarbons of a greater degree of saturation wherein said hydrocarbon mixture is contacted with sulfur dioxide in a reaction zone under conditions resulting in the formation of reaction products predominating in diolefin sulfones consisting essentially of isoprene sulfone in admixture with sulfur dioxide and unreacted hydrocarbons and said reaction products are heated in a decomposition zone at a temperature sufficiently high to decompose said diolefin sulfones with the liberation of isoprene, the steps which comprise passing reaction products from the reaction zone into a vaporizing zone, separating a vapor fraction comprising sulfur dioxide and unreacted hydrocarbons from a liquid fraction comprising diolefin sulfones, sulfur dioxide and residual unreacted hydrocarbons in said vaporizing zone, stripping said liquid fraction with sulfur dioxide in a stripping zone to remove substantially all unreacted hydrocarbons therefrom in the absence of any undesirable side reactions to any substantial degree, passing diolefin sulfones free of any substantial amount of residual unreacted hydrocarbons from the stripping zone to the decomposition zone, contacting effluence from said decomposition zone comprising isoprene and sulfur dioxide with a selective solvent for said diolefins in an extraction zone, thereby selectively absorbing isoprene in said solvent, separating isoprene from the resulting rich solvent containing isoprene and passing sulfur dioxide from the extraction zone to the stripping zone.

4. In a process for recovering piperylene from a hydrocarbon mixture comprising piperylene in admixture with hydrocarbons of a greater degree of saturation wherein said hydrocarbon mixture is contacted with sulfur dioxide in a reaction zone under conditions resulting in the formation of reaction products predominating in diolefin sulfones consisting essentially of piperylene sulfone in admixture with sulfur dioxide and unreacted hydrocarbons and said reaction products are heated in a decomposition zone at a temperature sufficiently high to decompose said diolefin sulfones with the liberation of piperylene, the steps which comprise passing reaction products from the reaction zone into a vaporizing zone, separating a vapor fraction comprising sulfur dioxide and unreacted hydrocarbons from a liquid fraction comprising diolefin sulfones, sulfur dioxide and residual unreacted hydrocarbons in said vaporizing zone, stripping said liquid fraction with sulfur dioxide in a stripping zone to remove substantially all unreacted hydrocarbons therefrom in the absence of any undesirable side reactions to any substantial degree, passing diolefin sulfones free of any substantial amount of residual unreacted hydrocarbons from the strip- 11 i2 Ding zone to the decomposition zone, contacting efliuence from said decomposition zone compris- REFERENCES CITED ing piperylene and sulfur dioxide with a selective The ll w references are of record in the solvent for said diolefins in an extraction zone, file of this patent: thereby selectively absorbing piperylene in said 5 solvent, separating piperylene from the result- UNITED STATES PATENTS ing rich solvent containing piperylene and pass- Number Name Date ing sulfur dioxide from the extraction Zone to 2,356,840 Frey et a1. Aug. 29, 1944 the stripping zone. 2,384,378 Hooker et a1 Sept. 4, 1945 JOSE A. SAMANIEGO. 10 2,399,837 Upham May '1, 1946 MOTT SOUDERS, JR. 

